Next up, a case of life imitating art. A few months ago, we talked to writer Barbara Kingsolver about her latest book, "Flight Behavior." The book is a fictional account of an ecological disaster in the making, and the fate of millions of monarch butterflies is at the center of the plot. Would the species survive? That's the art part.

Now comes the life imitating part. My next guest says that monarchs, in real life, are in serious trouble. The latest census from the butterflies' over-wintering spot in Mexico shows a population plunged this year. Habitat destruction, among other factors, is to blame.

So how will this story end? Joining me now to talk about it is my guest. Lincoln Brower is a professor emeritus of zoology at the University of Florida and a professor of biology at Sweet Briar College in Sweet Briar, Virginia. Thanks for talking to us today.

DR. LINCOLN BROWER: Well, thank you very much for having me. It's a wonderful opportunity.

Tell us - give us a sort of status report on the butterflies this year.

BROWER: Well, we have a record that goes back for 20 years, and we've known their colonies in Mexico since 1976. And this past year, the colony size hit 2.94 acres, and that was - the highest year was 52 acres. So there's an enormous difference between the highs and the lows, and we're way below average. And the problem is, over the last decade, we've just been going down, down, down. And this year it hit such a low number that we're worried about their recovery and what it means in the future.

LICHTMAN: And the number of acres corresponds, I guess, to the number of butterflies, right?

BROWER: Well, the way we measure the butterflies and the way we were able to get at the actual numbers, when you're looking at a colony, there are so many butterflies you can't possibly count them. But there was a storm in 2010, and butterflies were dead on the ground up to three feet deep, and we were able to estimate that there are 50 million butterflies per hectare, which is about 20 million per acre.

So if we had two - three acres, that would be about 60 million butterflies left this year, and that's getting down to being a perilously low number, because in the big year, with 52 acres, that year there were more than a billion butterflies over-wintering in Mexico, more than a billion butterflies.

LICHTMAN: A billion, with a B.

BROWER: With a B.

LICHTMAN: That is - it's shocking.

BROWER: Well, there's another decline this year that was very shocking too. We had - in previous years, we had 17 sites with colonies. This year, eight of those sites had zero butterflies. So - and the rest of them had very few butterflies. There was only one colony that had a significant number. And my worry is that they're winking out, one by one, and they may not be able to recover.

LICHTMAN: So they're not making it to these over-wintering grounds. That's what...

BROWER: The numbers - well, there - would you like me to discuss why we think they're going down?

LICHTMAN: I would. We only have about a minute left now, but let's start with one of the reasons anyway.

BROWER: Well, OK. Well, one of the main reasons why the town is going down overtime is the degradation of the Oyamel Forest, this is where the forest - the beautiful forest, high up in the mountains, protects the butterflies like an umbrella and a blanket, keeps them from freezing. Even though they're in the tropics, they're in such high elevation that they're subject to weather storms, Rain, fleet snow, freezing. And in 2010, there was the big chill, and if you've been in the forest, it's like cutting holes in a blanket or chopping holes in an umbrella. Rain gets in. The butterflies get wet. And then when it freezes on early morning, they die from frost.

LICHTMAN: We're going to hear lots more about the butterflies and what's causing the decline and what we can do when we come back from the break. Talking with Lincoln Brower, professor emeritus of zoology at the University of Florida and a professor of biology at Sweet Briar College in Virginia. Stay with us. This is SCIENCE FRIDAY from NPR.

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LICHTMAN: This is SCIENCE FRIDAY. I'm Flora Lichtman. We're talking this hour about the plight of the monarch butterfly whose numbers plunged this year and have been steadily declining in the last decade or so. We're speaking with Lincoln Brower, a professor emeritus of zoology at the University of Florida and a professor of biology at Sweet Briar College. And if you want to get in on the conversation, our number is 1-800-989-8255, 1-800-989-TALK. Well, Dr. Brower, let's start with some basic biology of what the butterflies are actually down in Mexico. What's their life cycle like?

BROWER: OK. Well, I need to get to the herbicide tolerant soybeans and corn crops at some point.

LICHTMAN: We will get there.

BROWER: OK, OK. Well, what is so amazing is that in the fall, or starting around the 21st of September, these million, up to, probably, two billion butterflies migrate out of their breeding range in the great lake states in the whole Eastern United States. And they migrate down through Texas, and they arrive in Mexico, almost to the day, on the 1st of November. And then they cluster in these beautiful fir trees. They sort of like the Christmas trees that we have, and it's called the Oyamel Fir Forest, And they become very, very tightly aggregated in these colonies, and they spend the entire winter there. And then when the 21st of March comes, which is exactly six months later, they leave Mexico and they fly North in Texas, breeding along the way. And when they get to Texas, they find the milkweeds, and there, they lay their eggs and establish the new spring generation.

So these butterflies are now seven or eight months old, having been born in September, spent the entire time in Mexico, hibernating there, and then moving back into the breeding range in the spring. It's a pretty amazing life. And then those offspring that are born in Texas take about three weeks to develop, and they migrate up North and getting all the way up into New England and then to Southern Ontario and establish the next generation. And then there's a couple of more generations. And then what is amazing is that the butterflies that are produced the following September are able to fly back almost exactly the same trees in Mexico that they've never seen before.

LICHTMAN: It's amazing. And the other thing that I wanted to ask you about is why the generation times are different? Some live for seven or eight months and the next generation lives for a fewer?

BROWER: Right. Well, this is a super generation that's produced in the fall, and it all relates back to the evolution of the monarch and its history going back, probably, a couple of million years. And that is the monarchs belong to a family of butterflies that are all tropical. But when milkweeds evolved in North America, now there are 108 species, this was a huge evolutionary opportunity and the monarchs migrated out of Mexico to take advantage of these milkweeds. But the monarchs are tropical butterflies, and so you have to migrate back to the tropics in order to survive.

So that's why we have this complex migration. And if they were to breed in Mexico, they would lose their migratory urge. So they have to stay quiet for several months and then fly back into Texas, get back into the breeding mode and establish the short-lived generations which then breed over the summer.

LICHTMAN: Looks like a relay, a baton being (unintelligible).

BROWER: It is. Exactly.

LICHTMAN: Well, so why is the milkweed so important to them? What do they use it for?

BROWER: Well, milkweed is the only plant that the whole family of butterflies can eat, and the monarch feeds on all these different species of milkweeds, and only a few of them are really common. And the problem is that milkweeds are being killed off by herbicides and genetically engineered soybean and corn crops on a huge scale.

LICHTMAN: Describe that more. How are they being killed of by the genetically-modified crops?

BROWER: Well, soybeans and corn have been genetically engineered to be resistant to high-powered herbicides, particularly Glyphosate which, I think, is also called Round-Up. And when a milkweed plant is sprayed with that, it kills it. When the corn and soybeans are sprayed with it, they're perfectly happy. They've been selected to be resistant to the herbicide.

So we know that 63 percent of the milkweeds in the Midwestern United States - soybean and corn belt - have been killed over the past decade. And I, who's been out there and seen these - I was looking at a soybean field, and I can only describe it like standing on the fore deck of the Queen Mary looking out over the sea of soybean, and there was not a single milkweed growing or any other plants growing in that entire field. So these herbicides are sterilizing the environment on a scale which is probably unprecedented in the history of humanity.

LICHTMAN: It seems like there are a lot of economic incentives for this type of agriculture, and it would be hard to change the direction of that train overnight. But is there something that people like our listeners could do tomorrow to help the monarchs?

BROWER: Well, Xerces Society, X-E-R-C-E-S Society and Monarch Watch and other groups are developing milkweed gardens and encouraging people to plant them extensively throughout the range of the butterflies. It's a partial mitigation. Another mitigation that could be gotten into that I think would be very significant is to work with highway departments in all the different states so they don't spray the sides of the roads or cut them at the wrong time. And we know if you cut the milkweeds at the right time, it actually promotes the growth of the caterpillars.

LICHTMAN: Well, that's a great idea. So all of our highways could be lined with milkweed.

BROWER: And the midways too. I mean, it would be - it would not only be milkweeds. And another really important aspect of this (unintelligible) say kill off, is that it kills all the nectar plants, and so that the - all insects and all the birds, the entire food web is affected by these kills. Whereas if we had wildflowers and milkweeds growing all over the roads along the United States, it would be a significant mitigation.

LICHTMAN: You can go to sciencefriday.com/milkweed to learn more about where to get seeds if you want to plant your own monarch garden. And they can still seek it out if you put it in your garden?

BROWER: Oh, absolutely. Monarchs are absolutely magicians when it comes to finding milkweeds. In fact, one time, we were making a movie with some people and the milkweeds had barely come up and everybody was very worried, couldn't find any, and I said follow that female. And she just went from plant to plant to plant and finally we found all the plants in the field. That's what's pretty impressive.

TOM: Oh, I'm just listening to your show, and I am glad this came up, this subject came up. I grew up on the northwest side of Milwaukee in the late '50s and the early '60s, and there was a forest. It was pretty rural back in those days. There was a forest, and as children, we would go into this little forest and we'd see thousands of monarchs. It was amazing. We were in just in awe of what we saw. At about 1963 and '64, homes were built in that same spot, and they tore the trees down.

There are no longer farms there. There are no longer the monarchs there. And I - my statement is probably - along the migration highway, some of the habitat along that way is also being destroyed, too, which kind of helps with the endangerment of their species.

LICHTMAN: Thanks for calling, Tom.

TOM: OK. Thank you very much.

LICHTMAN: Did you want to say something, Dr. Brower?

BROWER: Well, he's absolutely right. I mean, the problem is the habitat destruction on a grand scale. And development is another problem, just taking what were natural fields and converting them into development. But I think the really serious problem that we have to confront, I think we have to work with the chemical companies to help us mitigate this business of killing off the entire food chain. It's just a very serious problem.

LICHTMAN: I want to mention another link. If you want to track your monarch sightings, go to sciencefriday.com/monarchmath. And if you're interested in milkweed: sciencefriday.com/milkweed. Let's go back to the phones because a lot of people want to talk about this. Ashley(ph) in Indianapolis. Welcome to Science Friday.

ASHLEY: Thank you for taking my call. My question is about how climate change, global warming could possibly be playing a role with the disappearance of the butterflies.

LICHTMAN: Did you get that, Dr. Brower? How...

BROWER: Yes. Well, there's a lot of evidence that's coming out to indicate that monarchs - butterflies that occur in the mountains are moving higher where it's cooler and moving further north. With the monarch, we don't know what's going to happen. It's possible if the milkweeds will move north, the monarch is such a mobile creature, it may be able to follow them.

I think the problem is in Mexico if - there are several studies that suggest that this beautiful oyamel forest will be driven above its capability of surviving, and so that may be a problem. But again, the monarchs are mobile, and they may be able to move if the forests move.

LICHTMAN: Is there any chance of monarchs relocating their overwintering sites somewhere north of Mexico?

BROWER: This was a question that my colleague, Dan Slayback, at NASA had, and we flew over the area for about two weeks looking for colonies in the whole area. We covered a huge area. And we did not find a single new site other than the ones that we already had known about. I was astounded. I had predicted - the were lots of forest, they should be there, and they weren't. They were always going back to the same spot.

LICHTMAN: Do we have an idea of what's so great about those little areas or what's different about them?

BROWER: Well probably they may very well be very old. And we have a hypothesis that the monarchs are, in some way, marking those spots with a chemical that persists through the season to the next fall. And when the new generation comes back, somehow they sense that odor or chemical, and that's why they're always coming back to the same places.

And one of the supports to the hypothesis is the areas which have been terribly degraded by predatory logging. The monarchs have come back to them even though they are super exposed and really vulnerable to freezing.

LICHTMAN: Hmm. And that would explain why the different generations that have never been there before can find the right spot.

BROWER: Exactly. A lot of people don't agree with this. And we have very weak evidence for support of it, but I think it will prove to be the case as time goes on.

LICHTMAN: I'm talking with Lincoln Brower, professor emeritus of zoology at University of Florida and professor of biology at Sweet Briar College. You're listening to SCIENCE FRIDAY on NPR. I'm Flora Lichtman.

So you've been studying these guys for over 50 years now? How do you not just get so depressed by this news?

BROWER: Well, I think underneath it all I am an optimist. And one of my - one of my wonderful reasons to be on this program is to reach people because we need a constituency who are going to stand up for all wildlife. And the monarch is really popular with school teachers. Journey North have something like 40,000 participants. Monarch Watch has at least that many. And it's wonderful educational tool, and it's such a beautiful thing for anybody to get to Mexico to see it. It's just overwhelming to see it. And to me, the monarch is a treasure like a great piece of art, and that we really need to develop a cultural appreciation of wildlife that's equivalent to art and music and so forth.

LICHTMAN: While you see pictures of these ever-wintering sites and it's spectacular even just through the Internet.

BROWER: They are spectacular.

LICHTMAN: Let's go to Des Moines. Kay(ph), welcome to SCIENCE FRIDAY.

KAY: Thank you so much for taking my call. Thank you for mentioning Journey North. I've been watching - we've been following that for two years. My kids and I haven't harvesting the eggs and then raising them inside and then releasing them. And I've often wondered and didn't have anyone to ask - are we helping? Are we hurting (unintelligible) the pollutants water and to protect and we'll share it the classrooms so more kids have a chance to watch (unintelligible) and emerge from their chrysalis. So if you're doing this or is this damaging the population?

LICHTMAN: Thanks, Kay. What do you think...

BROWER: Well, my answer to that is anything you can do to get children interested in the monarch, even if there is a slight cost, the benefit is going to way outweigh that cost and those will be the people that are going to protect the butterfly on into the future.

LICHTMAN: What are the butterflies' natural predators?

BROWER: Well, that was in - lot of my early work was done with that my wife, Linda Fink, and we fond in Mexico that there are two major predators. And it's a really interesting story because milkweeds are toxic plants and the butterflies absorb and store the toxin into their bodies. And if a blue jay eats a monarch butterfly that fed on a toxic milkweed it'll throw up.

LICHTMAN: You took that famous picture of the blue jay throwing up.

BROWER: That's Brower's barfing blue jay.

(LAUGHTER)

BROWER: Anyway, and what we found in Mexico was that there were two species of birds, and oriole and grosbeak, which had broken through the chemical defense. And it turns out that the orioles were really clever. They were pecking into the butterfly and tasting it. And if it was poisonous, they dropped it because it turns out that only about maybe 10 percent of the butterflies in Mexico are really toxic. And so they are, by taste discrimination, able to break through this chemical defense. And so that sort of gets you to another philosophical point that no defense is ever perfect. Somebody is always going to break through it.

LICHTMAN: Hmm. Yeah, I like that note. I wonder, too, I read a little bit about dragonflies. Are they predators? I read that they - OK. I'll just say it. I read that they ate the faces off of butterflies. Is that true?

BROWER: Well, the cripsota praying mantises, but I think the overall mortality by praying - by dragonflies is probably not that large and...

LICHTMAN: Go ahead.

BROWER: Well, the big dangers to the monarch is loss of habitat. And I think that all of the other mortality factors are pretty minor.

LICHTMAN: Well, thank you so much for joining us today and telling us about this, Dr. Brower.

BROWER: Thank you, Flora, for asking wonderful questions.

LICHTMAN: Have a great weekend.

BROWER: Thank you.

LICHTMAN: Lincoln Brower is a professor emeritus of zoology at the University of Florida and a professor of biology at Sweet Briar College in Sweet Briar, Virginia. Transcript provided by NPR, Copyright NPR.